Catalysts Wilkinsons

In 1965 Wilkinson invented the rhodium-tris(triphenylphosphine) catalyst as a hydrogenation catalyst [60]. It still forms the basis for many of the chiral hydrogenations performed today. The most effective homogeneous hydrogenation catalysts are complexes consisting of a central metal ion, one or more (chiral) ligands and anions which are able to activate molecular hydrogen and to add the two H atoms to an acceptor substrate. Experience has shown that low-valent Ru, 

Rh and Ir complexes stabilized by tertiary (chiral) phosphorus ligands are the most active and the most versatile catalysts. Although standard hydrogenations of olefins, ketones and reductive aminations are best performed using heterogeneous catalysts (see above), homogeneous catalysis becomes the method of choice once selectivity is called for. An example is the chemoselective hydrogenation of a,/ -unsaturated aldehydes which is a severe test for the selectivity of catalysts. 

The catalytic cycle starts with the dissociation of one ligand P which is replaced e.g. by a solvent molecule. An oxidative addition reaction of dihydrogen 

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Complex formation with the Wilkinson catalyst RhCl(PPh3)3 gives 351 [870M(319)311]. The reaction with [Rh(Ti -C2H4)2Cl]2 gives the triple-decker... 

In 1971, a short communication was published [54] by Kumada and co-workers reporting the formation of di- and polysilanes from dihydrosilanes by the action of a platinum complex. Also the Wilkinson catalyst (Ph3P)3RhCl promotes hydrosilation. If no alkenes are present, formation of chain silanes occurs. A thorough analysis of the product distribution shows a high preference for polymers (without a catalyst, disproportionation reactions of the silanes prevail). Cross experiments indicate the formation of a silylene complex as intermediate and in solution, free silylenes could also be trapped by Et3SiH [55, 56],... 

Complex 5 was more active than the well-known precious-metal catalysts (palladium on activated carbon Pd/C, the Wilkinson catalyst RhCl(PPh3)3, and Crabtree s catalyst [lr(cod)(PCy3)py]PFg) and the analogous Ai-coordinated Fe complexes 6-8 [29] for the hydrogenation of 1-hexene (Table 2). In mechanistic studies, the NMR data revealed that 5 was converted into the dihydrogen complex 9 via the monodinitrogen complex under hydrogen atmosphere (Scheme 4). 

A series of anchored Wilkinson s catalysts were prepared by reacting the homogeneous Wilkinson catalyst with several alumina/heteropoly acid support materials. These catalysts were used to promote the hydrogenation of 1-hexene. The results were compared with those obtained using the homogeneous Wilkinson and a l%Rh/Al203 catalyst with respect to catalyst activity and stabihty as well as the reaction selectivity as measured by the amount of double bond isomerization observed. The effect which the nature of the heteropoly acid exerted on the reaction was also examined. 

From these data it is obvious that the supported Rh metal is much more active than the homogeneous or anchored Wilkinson catalysts and that the initial rate... 

Another interesting comparison of the homogeneous Wilkinson catalyst with AHC-Wilk is in the high TON hydrogenation of cyclohexene in 10%... 

The AHC Wilkinson catalyst (Rh(Ph3P)3/PTA/Al203) (AHC-Wilk) was prepared using the general procedure described previously (10,12). The catalyst contained 0.5% Rh which corresponds to a 1 1 Rh PTA ratio and about a 4.5% load of the anchored complex. The hydrogenations were ran using the low pressure apparatus previously described (19) under the conditions listed in the discussion. 

A similar reactor setup was used by Keurentjes et /. 9,10 A Wilkinson catalyst with fluorinated ligands was applied in the hydrogenation of 1-butene in supercritical... 

Mechanistic details for hydrogenation of ethylene and cyclohexene catalyzed by the well-known Wilkinson catalyst, RhCl(PPh3)3 (7, p. 204) have been further elucidated (69-74) (Fig. 1). Studies on the analogous... 

The review of Morrison et al. (10) traces the development of the use of rhodium-chiral phosphine catalysts to about the end of 1974. This field was initiated by the suggested incorporation (216) of chiral phosphines, instead of triphenylphosphine, into the so-called Wilkinson catalyst, RhCl(PPh3)3 (Section II,A), or into closely related systems. Horner s group (217, 218) used such catalysts, formed in situ in benzene... 

RCH/RP [Ruhrchemie/Rhone Poulenc] A variation of the 0X0 process in which the triphenyl phosphine (part of the Wilkinson catalyst) is sulfonated, in order to render the catalyst soluble in water for easier recovery. First commercialized in 1984 for the manufacture of butyraldehyde. 

A ketimine can also be alkylated by the same process.140 In situ generation of a ketimine from the aromatic ketone 114 and benzylamine provides an efficient catalytic process with Wilkinson catalyst (Scheme 35). The alkylated aromatic ketone 115 is obtained in good yield. Better reactivity and selectivity are obtained with ketimine... 

Intramolecular process with rhodium catalyst has been described for the syntheses of indane, dihydroindoles, dihydrofurans, tetralins, and other polycyclic compounds. Wilkinson catalyst is efficient for the cyclization of aromatic ketimines and aldimines containing alkenyl groups tethered to the K z-position of the imine-directing group. 

The hydride route involves the initial reaction with hydrogen followed by coordination of the substrate the well-known Wilkinson catalyst [RhCl(PPh3)3] is a representative example. A second possible route is the alkene (or unsaturated) route which involves an initial coordination of the substrate followed by reaction with hydrogen. The cationic catalyst derived from [Rh(NBD)(DIPHOS)]+ (NBD = 2,5-norbornadiene DIPHOS = l,2-bis(diphenyl)phosphinoethane) is a well-known example. The above-mentioned rhodium catalysts will be discussed, in the detail, in the following sections. 

Scheme 20.8 Different behavior of the Wilkinson catalyst (7) for transfer hydrogenation and hydrogenation using molecular hydrogen.

Scheme 20.10 Racemization of (S)-l-deutero-l-phenylethanol (8) with the Wilkinson catalyst (7).

Fig. 20.1 Examples of catalysts operating via the same mechanism as the Wilkinson catalyst (bipy= bipyridine dppp = l,3-bis(diphenylphosphinopropane) [35, 44, 45].

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